Optical communications apparatus

ABSTRACT

A large, high-speed optical communications apparatus is provided capable of selecting a signal transmission route in optical domain. An address extractor  101  extracts address information from data information included in a transmission signal. A variable frequency RF modulator  102  modulates the data information into an RF modulated signal having a predetermined frequency that corresponds to a lower address. A variable wavelength optical modulator  103  modulates the RF modulated signal into an optical signal having a predetermined wavelength that corresponds to an upper address. An optical router  105  outputs the optical signal according to the optical wavelength. A first RF optical router  1071  outputs the optical signal from a first or second output terminal provided thereto according to the RF modulating frequency. A second RF optical router  1072  operates similarly. First to fourth optical receivers  1091  to  1094  each converts the optical signal coming from the corresponding output terminal of the first or second RF optical router  1071, 1072  into an electrical signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to optical communicationsapparatuses and, more specifically, to an optical communicationsapparatus for transmitting an optical signal by switching opticalcommunications paths based on the wavelength and modulating frequency ofthe optical signal as address information.

[0003] 2. Description of the Background Art

[0004]FIG. 10 is a block diagram showing the structure of a conventionaloptical communications apparatus. One example of such apparatus isdisclosed in detail in “Hyperspace Addressed Optical Access Architectureusing Active Arrayed Waveguide Gratings”, F. Farjaday, M. C. Parker, andS. D. Walker, OECC98, 15A2-2, 1998.

[0005] In FIG. 10, the optical communications apparatus includes anoptical transmitting circuit 10001, a main optical transmission path1004, an optical router 1005, first and second distribution opticaltransmission paths 10061 and 10062, and first and second opticalreceiving circuits 10091 and 10092. The optical transmitting circuit10001 includes an address extractor 1010 and a variable wavelengthoptical modulator 1003.

[0006] In the above structured optical communications apparatus, theaddress extractor 1010 extracts, from a signal received by the opticaltransmitting circuit 10001, address information indicating thedestination to which the signal should go. Alternatively, the addressextractor 1010 may be provided with the address information itselfseparately.

[0007] The variable wavelength optical modulator 1003 is composed of avariable wavelength light source capable of changing the wavelength ofoutput light. This wavelength is uniquely determined based on theaddress information extracted by the address information extractor 1010or separately provided. The variable wavelength optical modulator 1003optically modulates the signal including the above described datainformation, and then sends out light having the determined wavelengthto the main optical transmission path 1004.

[0008] The optical router 1005, exemplarily composed of an AWG (ArrayedWaveGuide), has a plurality of output terminals (in this example, firstand second output terminals 10051 and 10052) for selectively outputtingthe optical signal based on the wavelength of the input light. Whensupplied with the optical signal through the main optical transmissionpath 1004, the optical router 1005 outputs it from the first terminal10051 when the optical wavelength thereof is λ1, while outputting fromthe second terminal 10052 when λ2.

[0009] The first and second optical receiving circuits 10091 and 10092are each connected to the optical router 1005 at the first outputterminal 10051 and at the second output terminal 10052, respectively.The first and second optical receiving circuits 10091 and 10092 eachconvert the optical signal from each corresponding output terminal intoan electrical signal for output.

[0010] As described above, in the conventional optical communicationsapparatus, a variable wavelength light source is used as the lightsource in the optical transmitting circuit to control the wavelength ofthe output light based on the address information indicating thedestination to which the data information should go. Also, the opticalrouter is provided on the optical transmission path, enabling routing ofthe input light for output from each different terminal based on thewavelength thereof. Thus, the conventional optical communicationsapparatus can carry out autonomous switching among the transmissionpaths in optical domain, and therefore a high-speed opticalcommunications network can be achieved.

[0011] One disadvantage here is, when the wavelength of the opticalsignal is used as an address, the number of wavelengths or wavelengthbands available on the optical transmission path is limited. Thisdisadvantage is described below with reference to FIG. 11.

[0012]FIG. 11 is a schematic diagram demonstrating the limitation of thenumber of wavelengths in the conventional optical communicationsapparatus. Specifically, as shown in FIG. 11, Erbium-doped fiber opticalamplifiers (EDFA) widely used in optical transmission systems cangenerally carry out amplification only within approximately 30 to 40 nmin a wavelength band of 1.55 μm. On the other hand, AWGs and opticalfilters generally have a wavelength resolving power (dividable opticalwavelength period) of approximately 0.8 nm. In FIG. 11, the band passcharacteristics of the optical filter is represented as a dotted line.Consequently, the number of wavelengths available in address space isvery limited, approximately 40 to 50. Thus, in the conventional opticalcommunications apparatus, the number of optical receiving terminals thatcan be connected thereto or covered thereby (the number of subscribers)is disadvantageously limited, and a large optical communications networkcannot be constructed.

[0013] In order to construct a large optical communications networkusing the conventional optical communications apparatus, one structurecan be suggested, where further routing is made using electrical signalsoutputted from the first and second optical receiving circuits 10091 and10092 for transmitting information to end receiving terminals(subscribers). In such structure, however, unauthorized informationextraction and tampering are highly possible due to the use of theelectrical signals for information transmission to the end receivingterminals (subscribers), compared to the case where optical signals areused. Also, conventional communications networks using electricalsignals are inferior, in transmission speed and amount of transmittableinformation, to optical communications networks using optical signalsfor transmitting information up to end users.

SUMMARY OF THE INVENTION

[0014] Therefore, an object of the present invention is to provide anoptical communications apparatus achieving a large opticalcommunications network with high speed and security by using thewavelength of an optical signal as an address for switching amongtransmission paths in optical domain.

[0015] The present invention has the following features to achieve theobject above.

[0016] A first aspect of the present invention is directed to an opticalcommunications apparatus for optically transmitting a transmissionsignal including data information a destination, and the apparatusincludes:

[0017] a variable frequency RF modulator for modulating the transmissionsignal into an RF modulated signal, with a predetermined carrierfrequency that corresponds to a lower address of address informationuniquely set to the destination, the lower address representing thedestination in a predetermined group to which the destination belongs;

[0018] a variable wavelength optical modulator for modulating the RFmodulated signal outputted from the variable frequency RF modulator intoan optical signal, with a predetermined optical wavelength thatcorresponds to an upper address of the address information, the upperaddress representing the predetermined group to which the destinationbelongs;

[0019] an optical router provided with a plurality of output terminals,for selectively outputting the optical signal outputted from thevariable wavelength optical modulator from one of the output terminalsthat corresponds to the wavelength of the optical signal;

[0020] a plurality of RF optical routers each provided with a pluralityof output terminals, for selectively outputting the optical signalcoming from the output terminal of the optical router from one of theoutput terminals that corresponds to the carrier frequency of the RFmodulated signal on the optical signal; and

[0021] a plurality of optical receiving circuits each for converting theoptical signal outputted from the corresponding output terminal of theRF optical router into an electrical signal that corresponds to thetransmission signal.

[0022] In the first aspect, by using the structure capable of selectinga signal transmission route in optical domain for switching (routing),the optical wavelength is related to the upper address of the addressinformation indicative of the signal destination, and the (carrier)frequency of the RF modulated signal is related to the lower address.Based on the optical wavelength, a first optical routing is carried out,and then, based on the RF modulating frequency, a second optical routingis carried out. Thus, a large-capacity, high-speed opticalcommunications apparatus capable of covering more optical receivingterminals can be achieved.

[0023] According to a second aspect, in the first aspect, the apparatusfurther includes an address extractor for extracting the addressinformation from the transmission signal including the addressinformation, and outputting the lower address to the variable frequencyRF modulator and the upper address to the variable wavelength opticalmodulator.

[0024] In the second aspect, the transmission signal further includesaddress information in addition to data information. Therefore, byextracting the address information from the transmission signal foroptical routing, the optical communications apparatus does not have tobe separately supplied with the address information.

[0025] According to a third aspect, in the first aspect,

[0026] the variable frequency RF modulator is plurally provided, eachconverting the transmission signal to a different destination into theRF modulated signal with different carrier frequency,

[0027] the variable wavelength optical modulator is plurally provided,each converting the RF modulated signal outputted from the correspondingvariable frequency RF modulator into the optical signal, and

[0028] the optical router is supplied with the optical signals from allvariable wavelength optical modulators as being multiplexed.

[0029] In the third aspect, optical signals coming from a plurality ofoptical transmitting circuits are multiplexed, and in the opticalspectrum of the multiplexed optical signal, a transmission route isselected based first on the optical wavelength, and then on the RFmodulating frequency. Thus, the optical transmission path is moreefficiently used, and a high-speed, large-capacity optical multiplexcommunications apparatus can be achieved.

[0030] According to a fourth aspect, in the first aspect,

[0031] the variable wavelength optical modulator carries out opticalintensity modulation,

[0032] the variable frequency RF modulator carries out ASK (AmplitudeShift Keying) modulation,

[0033] each of the RF optical routers includes:

[0034] an optical brancher for outputting the optical signal from aplurality of output terminals; and

[0035] a plurality of optical modulators each for subjecting the opticalsignal outputted from the corresponding output terminal of the opticalbrancher to optical intensity modulation with a signal having afrequency equal to the carrier the predetermined frequency of the RFmodulated signal, and

[0036] each of the optical receivers includes:

[0037] a square-law-detector for carrying out square-law-detection onthe optical signal outputted from the corresponding RF optical router,and outputting an electrical signal; and

[0038] a filter for passing a predetermined low frequency component ofthe electrical signal outputted from the square-law-detector, andoutputting baseband information of the RF modulated signal.

[0039] In the fourth aspect, optical intensity modulation is used as theoptical modulation scheme, and ASK modulation is used as the RFmodulation scheme. The optical signal is modulated with the frequencycorresponding to the RF modulated signal to be extracted,square-detected by the optical receiving terminal, and then basebandinformation of the RF modulated signal is reproduced for routing in theoptical domain based on the RF modulated frequency. Thus, a larger,higher-speed optical communications apparatus can be achieved.

[0040] According to a fifth aspect, in the first aspect,

[0041] the variable wavelength optical modulator carries out opticalintensity modulation,

[0042] each of the RF optical routers includes:

[0043] an optical brancher for outputting the optical signal from aplurality of output terminals; and

[0044] a plurality of optical filters each for extracting, from theoptical signal outputted from the corresponding output terminal of theoptical brancher, an optical carrier component and a double sidebandcomponent corresponding to the predetermined frequency of the RFmodulated signal, and

[0045] each of the optical receivers includes:

[0046] a square-law-detector for carrying out square-law-detection onthe optical signal outputted from the corresponding RF optical router,and outputting the RF modulated signal.

[0047] In the fifth aspect, optical intensity modulation is used as theoptical modulation scheme. From the optical signal, the optical carriercomponent and the double sideband component corresponding to the RFmodulated signal to be extracted is passed and extracted,square-detected by the optical receiving terminal, and then the RFmodulated signal is reproduced for routing in the optical domain basedon the RF modulated frequency. Thus, a larger, higher-speed opticalcommunications apparatus can be achieved.

[0048] According to a sixth aspect, in the first aspect,

[0049] the variable wavelength optical modulator carries out opticalintensity modulation,

[0050] each of the RF optical routers includes:

[0051] an optical brancher for outputting the optical signal from aplurality of output terminals; and

[0052] a plurality of optical filters each for extracting, from theoptical signal outputted from the corresponding output terminal of theoptical brancher, double sideband components corresponding to thepredetermined frequency of the RF modulated signal, and

[0053] each of the optical receivers includes:

[0054] a square-law-detector for carrying out square-law-detection onthe optical signal outputted from the corresponding RF optical router,and outputting a signal component that is a multiplied component of theRF modulated signal.

[0055] In the sixth aspect, optical intensity modulation is used as theoptical modulation scheme. From the optical signal, the double sidebandcomponents corresponding to the RF modulated signal to be extracted arepassed and extracted, square-detected by the optical receiving terminal,and then the RF modulated signal is reproduced for routing in theoptical domain based on the RF modulated frequency. Thus, a larger,higher-speed optical communications apparatus can be achieved.

[0056] According to a seventh aspect, in the first aspect,

[0057] the variable wavelength optical modulator carries out opticalfrequency modulation,

[0058] each of the RF optical routers includes:

[0059] an optical brancher for outputting the optical signal from aplurality of output terminals; and

[0060] a plurality of optical filters each for suppressing, on theoptical signal outputted from the corresponding output terminal of theoptical brancher, any one of an upper sideband component and a lowersideband component corresponding to the predetermined frequency of theRF modulated signal, and

[0061] each of the optical receivers includes:

[0062] a square-law-detector for carrying out square-law-detection onthe optical signal outputted from the corresponding RF optical router,and outputting the RF modulated signal.

[0063] In the seventh aspect, optical intensity modulation is used asthe optical modulation scheme. In the optical signal, any one of thedouble sidebands corresponding to the RF modulated signal to beextracted is suppressed. Then, the optical signal is square-detected bythe optical receiving terminal, and then the RF modulated signal isreproduced for routing in the optical domain based on the RF modulatedfrequency. Thus, a larger, higher-speed optical communications apparatuscan be achieved.

[0064] An eighth aspect of the present invention is directed to anoptical communications method for optically transmitting a transmissionsignal including data information to a destination, and the methodincludes:

[0065] a variable frequency RF modulating step of modulating thetransmission signal into an RF modulated signal with a predeterminedcarrier frequency that uniquely corresponds to the destination in apredetermined group to which the destination belongs;

[0066] a variable wavelength optical modulating step of modulating theRF modulated signal outputted from the variable frequency RF modulatorinto an optical signal with a predetermined optical wavelength thatuniquely corresponds to the predetermined group to which the destinationbelongs;

[0067] an optical routing step of selecting a distribution routecorresponding to the wavelength of the optical signal converted in thevariable wavelength optical modulating step, and outputting the opticalsignal to the distribution route;

[0068] an RF optical routing step of selecting an end routecorresponding to the carrier frequency of the RF modulated signal of theoptical signal outputted in the optical routing step, and outputting theoptical signal to the end route; and

[0069] an optical receiving step of converting the optical signaloutputted in the RF optical routing step into an electrical signal thatcorresponds to the transmission signal.

[0070] These and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0071]FIG. 1 is a block diagram showing the structure of an opticalcommunications apparatus according to a first embodiment of the presentinvention;

[0072]FIG. 2 is a schematic diagram for demonstrating arrangement of thewavelengths of an optical signal and RF modulating frequencies in thefirst embodiment of the present invention;

[0073]FIG. 3 is a block diagram showing a first example of structure ofRF optical routers and optical receiving circuits in the opticalcommunications apparatus according to the first embodiment of thepresent invention;

[0074]FIG. 4 is a schematic diagram demonstrating the operationalprinciple of optical routing based on the RF modulated signal under thefirst example of structure of the optical communications apparatusaccording to the first embodiment of the present invention;

[0075]FIG. 5 is a block diagram showing a second example of structure ofthe RF optical routers and the optical receiving circuits in the opticalcommunications apparatus according the first embodiment of the presentinvention;

[0076]FIG. 6 is a schematic diagram demonstrating the operationalprinciple of optical routing based on the RF modulated signal under thesecond example of structure of the optical communications apparatusaccording to the first embodiment of the present invention;

[0077]FIG. 7 is a schematic diagram demonstrating the operationalprinciple of optical routing based on the RF modulated signal under athird example of structure of the RF optical routers and the opticalreceiving circuits in the optical communications apparatus according tothe first embodiment of the present invention;

[0078]FIG. 8 is a schematic diagram demonstrating the operationalprinciple of optical routing based on the RF modulated signal under afourth example of structure of the RF optical routers and the opticalreceiving circuits in the optical communications apparatus according tothe first embodiment of the present invention;

[0079]FIG. 9 is a block diagram showing the structure of an opticalcommunications apparatus according to a second embodiment of the presentinvention;

[0080]FIG. 10 is a block diagram showing the structure of a conventionaloptical communications apparatus; and

[0081]FIG. 11 is a schematic diagram demonstrating limitations of thenumber of wavelengths in the conventional optical communicationsapparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0082] (First embodiment)

[0083] With reference to FIG. 1, an optical communications apparatusaccording to a first embodiment of the present invention is describedbelow. As shown in FIG. 1, the optical communications apparatus realizescommunications between one transmitting circuit and four main opticalreceiving circuits. Specifically, the optical communications apparatusincludes an optical transmitting circuit 1001; a main opticaltransmission path 104; an optical router 105; first and seconddistribution optical transmission paths 1061 and 1062; first and secondRF optical routers 1071 and 1072; first, second, third, and fourth endoptical transmission paths 1081, 1082, 1083, and 1084; and first, secondthird and fourth optical receiving circuits 1091, 1092, 1093, and 1094.Furthermore, the optical transmitting circuit 1001 includes an addressextractor 101, a variable frequency RF modulator 102, and a variablewavelength optical modulator 103.

[0084] Next, the operation of the optical communications apparatusillustrated in FIG. 1 is described. The address extractor 101 extracts,from data information supplied to the optical transmitting circuit 1001,address information indicating the destination of the data information.Alternatively, the address extractor 101 may be supplied with theaddress information separately from the data information.

[0085] The variable frequency RF modulator 102 modulates the datainformation into an RF modulated signal with a carrier having afrequency uniquely determined corresponding to all or part of theaddress information supplied by the address extractor 101. Such RFmodulated signal is typified by a digital signal modulated with amodulation scheme such as PSK or ASK.

[0086] The variable wavelength optical modulator 103 modulates the RFmodulated signal received from the variable frequency RF modulator 102with light with its wavelength set to a predetermined one correspondingto all or part of the address information supplied by the addressextractor 101. Then, the variable wavelength optical modulator 103 sendsout the resultant optical modulated signal to the main opticaltransmission path 104.

[0087] Here, the relation between the address information and theoptical wavelength and RF modulating frequency of the transmissionsignal is more specifically described. Assume herein that the addressinformation extracted by the address extractor 101 is six bits,represented as “A5, A4, A3, A2, A1, A0”. A5 is a most significant bit(MSB), and A0 is a least significant bit (LSB). Upper three bitsincluding the MSB (A5, A4, and A3) represent an upper address indicativeof a general group to which the destination of the data informationbelongs and that covers a larger area. On the other hand, lower threeincluding the LSB (A2, A1, and A0) represent a lower address indicativeof a specific group that covers a smaller area, or an individualdestination itself.

[0088] Here, the variable wavelength optical modulator 103 sets thewavelength of the optical signal based on the upper address, while thevariable frequency RF modulator 102 sets the carrier frequency of the RFmodulated signal based on the lower address. Thus, the optical signalcan be routed through a general group based on the upper address, andthen transmitted to a specific group or directly to a destination basedon the lower address.

[0089] The optical router 105 is supplied with the optical signal comingthrough the main optical transmission path 104. When the opticalwavelength of the received optical signal is a wavelength λ1, theoptical router 105 sends out the optical signal from a first outputterminal 1051 to the first distribution optical transmission path 1061.When the optical wavelength thereof is a wavelength λ2, the opticalrouter 105 sends out the optical signal from a second output terminal1052 to the second distribution optical transmission path 1062.

[0090] The first RF optical router 1071 is provided correspondingly tothe first output terminal 1051 of the optical router 105. The first RFoptical router 1071 receives the optical signal coming through the firstdistribution optical transmission path 1061. When the RF modulatingfrequency of the received optical signal is a first frequency f1, thefirst RF optical router 1071 sends out the optical signal from a firstoutput terminal 10711 to the first end optical transmission path 1081.When the RF modulating frequency thereof is a second frequency f2, thefirst RF optical router 1071 sends out the optical signal from a secondoutput terminal 10712 to the second end optical transmission path 1082.

[0091] Similarly, the second RF optical router 1072 is providedcorrespondingly to the second output terminal 1052 of the optical router105. The second RF optical router 1072 receives the optical signalcoming through the second distribution optical transmission path 1062.When the RF modulating frequency of the received optical signal is thefirst frequency f1, the second RF optical router 1072 sends out theoptical signal from a first output terminal 10721 to the third endoptical transmission path 1083. When the RF modulating frequency thereofis the second frequency f2, the second RF optical router 1072 sends outthe optical signal from a second output terminal 10722 to the fourth endoptical transmission path 1084.

[0092] The first to fourth optical receiving circuits 1091 to 1094 arerespectively connected to the first to fourth end optical transmissionpaths 1081 to 1084, each carrying out square-law-detection on theoptical signal received through the corresponding optical transmissionpath for conversion into an electrical signal, and then outputting theelectrical signal.

[0093] As such, in the present optical communications apparatus,switching of the transmission routes is carried out as shown in FIG. 2.That is, in the optical spectrum of the optical signal generated bywavelength-multiplexing the optical signals each modulated with the RFmodulated signal, the switching is carried out based on opticalwavelengths fx, fy, and fz as the upper address, and then based onoptical wavelengths f1, f2, and f3 as the lower address. Thus, comparedwith the optical routing only based on the optical wavelength, moreaddress space can be ensured. Therefore, the optical communicationsapparatus can be so achieved as to have a large capacity and cover moresubscribers.

[0094] Next, with reference to FIGS. 3 and 4, a first example ofstructure and operation of the RF optical router and the opticalreceiving circuits is described in detail. FIG. 3 is a diagramspecifically illustrating the first example of structure of the RFoptical routers (the first and second RF optical routers 1071 and 1072in FIG. 1) and the optical receiving circuits (the first, second, third,and fourth optical receiving circuits 1091, 1092, 1093, and 1094 in FIG.1).

[0095] In FIG. 3, an RF optical router 3001 includes an optical brancher301, first and second local oscillation signal sources 3021 and 3022,and first and second optical intensity modulators 3031 and 3032. A firstoptical receiving circuit 30021 includes a square-law-detector 304 and afilter 305. A second optical receiving circuit 30022 is similar instructure to the first optical receiving circuit 30021, and thereforenot illustrated in detail.

[0096] Next, the operation of the RF optical routers and the opticalreceiving circuits shown in FIG. 3 is described. Assume herein that ASK(Amplitude Shift Keying) modulation is used as the RF modulation schemeapplied to the optical signal outputted from the optical transmittingcircuit and inputted to the RF optical router 3001.

[0097] Also assume that the optical signal supplied to the RF opticalrouter 3001 is a signal obtained by multiplexing a plurality of RFmodulated signals S1, S2, S3, . . . , Sk, . . . , SN with differentcarrier frequencies. Note that, in (a) of FIG. 4, −fN to +fNrepresenting optical frequencies indicate how much an optical carrierfrequency fx is increased or decreased. Therefore, the opticalfrequencies −fN to +fN are respectively equivalent to opticalfrequencies fx−fN to fx+fN. In FIG. 5, this optical signal isrepresented as fx[S1(f1), S2(f2), . . . , Sk(fk), . . . , SN(fN)].

[0098] In the present optical communications apparatus, when only one RFmodulated signal is used, such state cannot be observed as that aplurality of RF modulated signals are frequency-multiplexed as statedabove. The above state may occur, however, if the variable frequency RFmodulator 102 uses a plurality of RF modulated signals. Therefore, forconvenience in description, assume that the optical signal supplied tothe RF optical router 3001 is a signal obtained by multiplexing aplurality of RF modulated signals.

[0099] The optical brancher 301 in the RF optical router 3001 branchesthe input optical signal for output. The first local oscillation signalsource 3021 outputs a first local oscillation signal Lk having afrequency fk equal to the frequency of the RF modulated signal Sk of theoptical signal outputted from a first output terminal 30011 of the RFoptical router 3001. With this first local oscillation signal Lk, thefirst optical intensity modulator 3031 modulates one optical signalbranched by the optical brancher 301 for optical intensity modulation.The optical signal after optical intensity modulation is outputted fromthe first output terminal 30011 of the RF optical router 3001.

[0100] Similarly, the second local oscillation signal source 3022outputs a second local oscillation signal Lj having a frequency fj equalto the frequency of the RF modulated signal Sj of the optical signaloutputted from a second output terminal 30012 of the RF optical router3001. With second first local oscillation signal Lj, the second opticalintensity modulator 3032 modulates the other optical signal branched bythe optical brancher 301 for optical intensity modulation. The opticalsignal after optical intensity modulation is outputted from the secondoutput terminal 30012 of the RF optical router 3001.

[0101] The first optical receiving circuit 30021 is connected to thefirst output terminal 30011 of the RF optical router 3001. The firstoptical receiving circuit 30021 carries out square-law-detection on theoptical signal outputted from that terminal, demodulates the desired RFmodulated signal Sk in the optical signal, reproduces basebandinformation Sb corresponding thereto, and outputs a baseband signal.Such reproduction of the baseband signal is possible because the RFmodulated signal Sk to be demodulated is a signal modulated by the ASKmodulation technique.

[0102] The filter 305 passes only the baseband information Sb, andeliminates the other unwanted components for output. A frequencyspectrum of the signal outputted from the square-law-detector 304 isillustrated in (b) of FIG. 4, where a passband of the filter 305 isenclosed by a dotted line. As can be seen from the drawing, only thebaseband information Sb is passed by the filter 305 for output.

[0103] The above described operation is further explained by usingequations. An electric field Ein(t) of the optical signal supplied tothe first optical intensity modulator 3031 can be represented by thefollowing equation (1), $\begin{matrix}{{E_{in}(t)} = {A\sqrt{\begin{matrix}\left\{ {{S_{b1}{C_{0S}\left( {2\pi \quad f_{1}t} \right)}} + \ldots \quad + {S_{bk}\left( {{C_{0S}\left( {2\pi \quad f_{k}t} \right)} + \ldots \quad +} \right.}} \right. \\{\left. {S_{bN}{C_{0S}\left( {2\pi \quad f_{N}t} \right)}} \right\} \times {C_{0S}\left( {2\pi \quad f_{x}t} \right)}}\end{matrix}}}} & (1)\end{matrix}$

[0104] where A is an electric field amplitude, fx is an opticalfrequency (optical wavelength), f1, . . . , fk, . . . , and fN are RFmodulating frequencies, Sb1, . . . , Sbk, . . . , and SbN are levels(“1” or “0”) of RF modulated signals (ASK modulated signals).

[0105] The first optical intensity modulator 3031 intensity-modulatesthe optical signal with the local oscillation signal (sine wave) havingthe frequency fk equivalent to the frequency of the ASK modulated signalto be extracted, and outputs an optical signal Eout(t) represented asthe following equation (2). $\begin{matrix}\begin{matrix}{{E_{out}(t)} = {A\sqrt{\begin{matrix}\left\{ {{S_{b1}{C_{0S}\left( {2\pi \quad f_{1}t} \right)}} + \ldots \quad + {S_{bk}\left( {{C_{0S}\left( {2\pi \quad f_{k}t} \right)} + \ldots \quad +} \right.}} \right. \\{{\left. {S_{bN}{C_{0S}\left( {2\pi \quad f_{N}t} \right)}} \right\} \cdot {C_{0S}\left( {2\pi \quad f_{x}t} \right)}} \times {C_{0S}\left( {2\pi \quad f_{x}} \right)}}\end{matrix}}}} \\{= {A\sqrt{\begin{matrix}\left\{ {{S_{b1}{{C_{0S}\left( {2\pi \quad f_{1}t} \right)} \cdot {C_{0S}\left( {2\pi \quad f_{k}t} \right)}}} + \ldots \quad +} \right. \\{{S_{bk}{{C_{0S}\left( {2\pi \quad f_{k}t} \right)} \cdot {C_{0S}\left( {2\pi \quad f_{k}t} \right)}}} + \ldots \quad +} \\{\left. {S_{bN}{{C_{0S}\left( {2\pi \quad f_{N}t} \right)} \cdot {C_{0S}\left( {2\pi \quad f_{k}t} \right)}}} \right\} \times} \\{C_{0S}\left( {2\pi \quad f_{x}t} \right)}\end{matrix}}}}\end{matrix} & (2)\end{matrix}$

[0106] The square-law-detector 304 carries out square-law-detection onthe optical signal for conversion into an optical current for output. Inthis optical current, only a component Ir(t) is extracted as representedby the following equation (3), which corresponds to the second term inthe above equation (2), $\begin{matrix}\begin{matrix}{{{Ir}(t)} = {{RA}^{2}S_{bk}{{C_{0S}\left( {2\pi \quad f_{k}t} \right)} \cdot {C_{0S}\left( {2\pi \quad f_{k}t} \right)}}}} \\{= {\frac{{RA}^{2}S_{bk}}{2}\left\{ {1 + {C_{0S}\left( {4\pi \quad f_{k}t} \right)}} \right\}}}\end{matrix} & (3)\end{matrix}$

[0107] where R is optical-electrical conversion efficiency.

[0108] Here, the first term in the above equation (3) is equal to thebaseband information Sb of the ASK modulated signal Sk. Therefore, ifunwanted components are eliminated by the filter 305, only the basebandinformation of the desired ASK modulated signal can be extracted.

[0109] Next, with reference to FIGS. 5 and 6, a second example ofstructure of the RF optical routers and the optical receiving circuitsin the present optical communications apparatus is described. FIG. 5 isa diagram specifically illustrating the second example of structure ofthe RF optical routers (the first and second RF optical routers 1071 and1072 in FIG. 1) and the optical receiving circuits (the first, second,third, and fourth optical receiving circuits 1091, 1092, 1093, and 1094in FIG. 1).

[0110] In FIG. 5, an RF optical router 5001 includes the opticalbrancher 301, and first and second optical filters 5011 and 5012. Afirst optical receiving circuit 50021 includes a square-law-detector503. A second optical receiving circuit 50022 is similar in structure tothe first optical receiving circuit 50021, and therefore not illustratedin detail.

[0111] Next, the operation of the RF optical routers and the opticalreceiving circuits shown in FIG. 5 is described. Here, as shown in (a)of FIG. 6, the optical signal outputted from the optical transmittingcircuit to the RF optical router 5001 is a signal obtained bymultiplexing a plurality of RF modulated signals S1, S2, S3, . . . , Sk,. . . , SN with different carrier frequencies. In FIG. 5, this signal isrepresented as λx[S1(f1), S2(f2), . . . , Sk(fk), . . . , SN(fN)].

[0112] The optical brancher 301 provided in the RF optical router 5001branches the input optical signal for output. The first optical filter5011 can pass only desired optical frequency components. Illustrated in(b) of FIG. 6 is such an example of transmittance characteristics of thefirst optical filter 5011. As shown in the drawing, peaks oftransmittance appear at the optical frequencies fx, fx+fk, and fx−fk.

[0113] With such characteristics, when supplied with one optical signalbranched by the optical brancher 301 as shown in (a) of FIG. 6, thefirst optical filter 5011 passes only the optical carrier component anddouble sideband components of the RF modulated signal Sk for output froma first output terminal 50011 of the RF optical router 5001. Thespectrum of the optical signal outputted from the first output terminal50011 is illustrated in (c) of FIG. 6. As can been seen from thedrawing, optical frequency components represented by dotted lines aresuppressed by the first optical filter 5011.

[0114] Similarly, when supplied with the other optical signal branchedby the optical brancher 301, the second optical filter 5012 passes onlythe optical carrier component and double sideband components of the RFmodulated signal Sj for output from a second output terminal 50012 ofthe RF optical router 5001.

[0115] The first optical receiving circuit 50021 is connected to thefirst output terminal 50011 of the RF optical router 5001, and carriesout square-law-detection on the optical signal received therefrom forreproducing and outputting the desired RF modulated signal.

[0116] Next, with reference to FIG. 7, a third example of structure ofthe RF optical routers and the optical receiving circuits in the presentoptical communications apparatus is described. Here, the third exampleis the same in structure as the second example shown in FIG. 5, butdifferent therefrom in transmittance characteristics of the opticalfilters (the first and second optical filters 5011 and 5012).

[0117] The first optical filter 5011 can pass only a desired opticalfrequency component. Illustrated in (b) of FIG. 7 is one example of suchtransmittance characteristics of the first optical filter 5011. As shownin the drawing, peaks of transmittance appear at the optical frequenciesfx+fk and fx−fk. Therefore, the optical frequency interval between thepeaks of transmittance shown in (b) of FIG. 7 is twice wider than thatshown in (b) of FIG. 6. Thus, the first optical filter 5011 is easier tofabricate, compared with that of the second example requiring highaccuracy.

[0118] With such characteristics, when supplied with one optical signalbranched by the optical brancher 301 as shown in (a) of FIG. 7, thefirst optical filter 5011 passes only double sideband components of theRF modulated signal Sk for output from the first output terminal 50011of the RF optical router 5001. The spectrum of the optical signaloutputted from the first output terminal 50011 is illustrated in (c) ofFIG. 7.

[0119] Similarly, when supplied with the other optical signal branchedby the optical brancher 301, the second optical filter 5012 passes onlydouble sideband components of the RF modulated signal Sj for output fromthe second output terminal 50012 of the RF optical router 5001.

[0120] The first optical receiving circuit 50021 is connected to thefirst output terminal 50011 of the RF optical router 5001, and carriesout square-law-detection on the optical signal received therefrom forreproducing and outputting a multiplied (doubled) component Sk′(frequency 2 fk) of the desired RF modulated signal. The frequencyspectrum of the optical signal outputted from the first opticalreceiving circuit 50021 is illustrated in (d) of FIG. 7. As shown in thedrawing, the RF modulated signal Sk is reproduced as a beat signalhaving the frequency 2 fk.

[0121] Next, with reference to FIG. 8, a fourth example of structure ofthe RF optical routers and the optical receiving circuits in the presentoptical communications apparatus is described. Here, the fourth exampleis also the same in structure as the second example shown in FIG. 5, butdifferent therefrom in transmittance characteristics of the opticalfilters (the first and second optical filters 5011 and 5012).Furthermore, the variable wavelength optical modulator 103 in the fourthexample carries out optical frequency modulation.

[0122] The first optical filter 5011 can pass only a desired opticalfrequency component. Illustrated in (b) of FIG. 8 is one example of suchtransmittance characteristics of the first optical filter 5011. As shownin the drawing, the lowest transmittance appears at the opticalfrequency fx−fk. Therefore, unlike the case shown in (b) of FIG. 7, onlyone optical frequency is enough to be distinctive. Therefore, the firstoptical filter 5011 is easier to fabricate.

[0123] With such characteristics, when supplied with one optical signalbranched by the optical brancher 301 as shown in (a) of FIG. 8, thefirst optical filter 5011 passes the optical signal while suppressingone of double sideband components of the RF modulated signal Sk, foroutput from the first output terminal 50011 of the RF optical router5001. The spectrum of the optical signal outputted from the first outputterminal 50011 is illustrated in (c) of FIG. 8.

[0124] Similarly, when supplied with the other optical signal branchedby the optical brancher 301, the second optical filter 5012 passes theoptical signal while suppressing one of double sideband components ofthe RF modulated signal Sj, for output from the second output terminal50012 of the RF optical router 5001.

[0125] The first optical receiving circuit 50021 is connected to thefirst output terminal 50011 of the RF optical router 5001, and carriesout square-law-detection on the optical signal received therefrom forreproducing and outputting a doubled component Sk of the desired RFmodulated signal.

[0126] The frequency spectrum of the optical signal outputted from thefirst optical receiving circuit 50021 is illustrated in (d) of FIG. 8.As shown in the drawing, the RF modulated signal Sk is reproduced at thefrequency fk.

[0127] As described in the foregoing, in the present opticalcommunications apparatus, in the optical spectrum of the optical signalmodulated with the RF modulated signal, switching of the transmissionroutes is carried out based first on the optical wavelength, and then onthe RF modulating frequency. Thus, more address space and subscriberscan be ensured. Therefore, the high-speed optical communicationsapparatus can be so achieved as to cover more subscribers.

[0128] In the above, the operation has been described assuming that theapparatus is structured by one optical transmitting circuit and fouroptical receiving circuits. However, the number of optical receivingcircuits is not restrictive, and may be more. As for the opticaltransmitting circuit, the case where two optical transmitting circuitsare included in the optical communications apparatus will be describedlater. Furthermore, the number of output terminals in the optical routerand the RF optical router may be two or more. In other word, the numberof optical signals to be extracted by the optical router and the RFoptical router for route selection may be two or more.

[0129] (Second embodiment)

[0130] With reference to FIG. 9, an optical communications apparatusaccording to a second embodiment of the present invention is describedbelow. In FIG. 9, the optical communications apparatus realizescommunications between two transmitting circuits and four main opticalreceiving circuits. Specifically, the optical communications apparatusincludes first and second optical transmitting circuits 10011 and 10012;first and second main optical transmission paths 1041 and 1042; anoptical router 105; first and second distribution optical transmissionpaths 1061 and 1062; first and second RF optical routers 1071 and 1072;first, second, third, and fourth end optical transmission paths 1081,1082, 1083, and 1084; and first, second, third, and fourth opticalreceiving circuits.

[0131] Furthermore, the first optical transmitting circuit 10011includes a first address extractor 1011, a first variable frequency RFmodulator 1021, and a first variable wavelength optical modulator 1031.The second optical transmitting circuit 10012 includes a second addressextractor 1012, a second variable frequency RF modulator 1022, and asecond variable wavelength optical modulator 1032.

[0132] Next, the operation of the optical communications apparatus isdescribed. The optical communications apparatus according to the secondembodiment is similar to that according to the first embodiment.Therefore, in the present embodiment, blocks similar to those accordingto the first embodiment are each provided with the same referencenumeral, and not described herein. Mainly described below is thedifference therebetween.

[0133] In FIG. 9, the first optical transmitting circuit 10011 convertsan RF modulated signal supplied by the first variable frequency RFmodulator 1021 into an optical modulated signal having a wavelength λx,and sends out to the first main optical transmission path 1041.Similarly, the second optical transmitting circuit 10012 converts an RFmodulated signal supplied by the second variable frequency RF modulator1022 into an optical modulated signal having a wavelength λy, and sendsout to the second main optical transmission path 1042.

[0134] Here, different information cannot be simultaneously sent to thesame destination. Therefore, at one point in time, the wavelength λx ofthe optical signal outputted from the first optical transmitting circuit10011 has to be different from the wavelength λy of the optical signaloutputted from the second optical transmitting circuit 10012. Also, thefrequency fa of the RF modulated signal outputted from the firstvariable frequency RF modulator 1021 has to be different from thefrequency fb of the RF modulated signal outputted from the secondvariable frequency RF modulator 1022. Therefore, to control thewavelengths and frequencies as such, a controller (not shown) may beprovided to the optical communications apparatus.

[0135] The optical router 105 multiplexes the optical signals outputtedfrom the first and second optical transmitting circuits 10011 and 10012.Also, when the optical wavelength of the multiplexed signal is the firstwavelength λx, the optical router 105 outputs the signal from the firstoutput terminal to the first optical transmission path 1061. On theother hand, when the optical wavelength is the second wavelength λy, theoptical router 105 outputs the signal from the second output terminal tothe second distribution optical transmission path 1062.

[0136] Note that the first and second RF optical routers 1071 and 1072and the first to fourth optical receiving circuits 1091 to 1094 aresimilar in structure to those of the first to fourth examples ofstructure according to the first embodiment.

[0137] As described in the foregoing, in the present opticalcommunications apparatus, optical signals from a plurality of opticaltransmitting circuits are first multiplexed. Then, in the opticalspectrum of the optical signal modulated with the RF modulated signal,switching of the transmission routes is carried out based first on theoptical wavelength, and then on the RF modulating frequency. Thus,optical transmission paths can be more efficiently used, and ahigh-speed, large-capacity optical communications apparatus can beachieved.

[0138] In the above, the operation has been described assuming that theapparatus is structured by two optical transmitting circuits and fouroptical receiving circuits. However, the number of optical transmittingcircuits and the number of optical receiving circuits are notrestrictive, and may be more. Furthermore, the number of outputterminals in the optical router and the RF optical router may be two ormore. In other word, the number of optical signals to be extracted bythe optical router and the RF optical router for route selection may betwo or more.

[0139] While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

What is claimed is:
 1. An optical communications apparatus for opticallytransmitting a transmission signal including data to a destination,comprising: a variable frequency RF modulator for modulating saidtransmission signal into an RF modulated signal, with a predeterminedcarrier frequency that corresponds to a lower address of addressinformation uniquely set to said destination, said lower addressrepresenting said destination in a predetermined group to which saiddestination belongs; a variable wavelength optical modulator formodulating said RF modulated signal outputted from said variablefrequency RF modulator into an optical signal, with a predeterminedoptical wavelength that corresponds to an upper address of said addressinformation, said upper address representing said predetermined group towhich said destination belongs; an optical router provided with aplurality of output terminals, for selectively outputting the opticalsignal outputted from said variable wavelength optical modulator fromone of the output terminals that corresponds to the wavelength of theoptical signal; a plurality of RF optical routers each provided with aplurality of output terminals, for selectively outputting the opticalsignal coming from the output terminal of said optical router from oneof the output terminals that corresponds to the carrier frequency ofsaid RF modulated signal on the optical signal; and a plurality ofoptical receivers each for converting the optical signal outputted fromthe corresponding output terminal of said RF optical router into anelectrical signal that corresponds to said transmission signal.
 2. Theoptical communications apparatus according to claim 1, furthercomprising: an address extractor for extracting said address informationfrom the transmission signal including said address information, andoutputting said lower address to said variable frequency RF modulatorand said upper address to said variable wavelength optical modulator. 3.The optical communications apparatus according to claim 1, wherein saidvariable frequency RF modulator is plurally provided, each convertingthe transmission signal to a different destination into said RFmodulated signal with different carrier frequency, said variablewavelength optical modulator is plurally provided, each converting saidRF modulated signal outputted from the corresponding variable frequencyRF modulator into the optical signal, and said optical router issupplied with the optical signals from all variable wavelength opticalmodulators as being multiplexed.
 4. The optical communications apparatusaccording to claim 1, wherein said variable wavelength optical modulatorcarries out optical intensity modulation, said variable frequency RFmodulator carries out ASK (Amplitude Shift Keying) modulation, each ofsaid RF optical routers includes: an optical brancher for outputting theoptical signal from a plurality of output terminals; and a plurality ofoptical modulators each for subjecting the optical signal outputted fromthe corresponding output terminal of said optical brancher to opticalintensity modulation with a signal having a frequency equal to thepredetermined carrier frequency of said RF modulated signal, and each ofsaid optical receivers includes: a square-law-detector for carrying outsquare-law-detection on the optical signal outputted from saidcorresponding RF optical router, and outputting an electrical signal;and a filter for passing a predetermined low frequency component of theelectrical signal outputted from said square-law-detector, andoutputting baseband information of said RF modulated signal.
 5. Theoptical communications apparatus according to claim 1, wherein saidvariable wavelength optical modulator carries out optical intensitymodulation, each of said RF optical routers includes: an opticalbrancher for outputting the optical signal from a plurality of outputterminals; and a plurality of optical filters each for extracting, fromthe optical signal outputted from the corresponding output terminal ofsaid optical brancher, an optical carrier component and double sidebandcomponents corresponding to the predetermined frequency of said RFmodulated signal, and each of said optical receivers includes: asquare-law-detector for carrying out square-law-detection on the opticalsignal outputted from said corresponding RF optical router, andoutputting said predetermined RF modulated signal.
 6. The opticalcommunications apparatus according to claim 1, wherein said variablewavelength optical modulator carries out optical intensity modulation,each of said RF optical routers includes: an optical brancher foroutputting the optical signal from a plurality of output terminals; anda plurality of optical filters each for extracting, from the opticalsignal outputted from the corresponding output terminal of said opticalbrancher, double sideband components corresponding to the predeterminedfrequency of said RF modulated signal, and each of said opticalreceivers includes: a square-law-detector for carrying outsquare-law-detection on the optical signal outputted from saidcorresponding RF optical router, and outputting a signal component thatis a multiplied component of said RF modulated signal.
 7. The opticalcommunications apparatus according to claim 1, wherein said variablewavelength optical modulator carries out optical frequency modulation,each of said RF optical routers includes: an optical brancher foroutputting the optical signal from a plurality of output terminals; anda plurality of optical filters each for suppressing, on the opticalsignal outputted from the corresponding output terminal of said opticalbrancher, any one of an upper sideband component and a lower sidebandcomponent corresponding to the predetermined frequency of said RFmodulated signal, and each of said optical receivers includes: asquare-law-detector for carrying out square-law-detection on the opticalsignal outputted from said corresponding RF optical router, andoutputting said RF modulated signal.
 8. An optical communications methodfor optically transmitting a transmission signal including datainformation to a destination, comprising: a variable frequency RFmodulating step of modulating said transmission signal into an RFmodulated signal with a predetermined carrier frequency that uniquelycorresponds to said destination in a predetermined group to which saiddestination belongs; a variable wavelength optical modulating step ofmodulating said RF modulated signal outputted from said variablefrequency RF modulator into an optical signal with a predeterminedoptical wavelength that uniquely corresponds to said predetermined groupto which said destination belongs; an optical routing step of selectinga distribution route corresponding to the wavelength of the opticalsignal converted in said variable wavelength optical modulating step,and outputting the optical signal to the distribution route; an RFoptical routing step of selecting an end route corresponding to thecarrier frequency of said RF modulated signal of the optical signaloutputted in said optical routing step, and outputting the opticalsignal to the end route; and an optical receiving step of converting theoptical signal outputted in said RF optical routing step into anelectrical signal that corresponds to said transmission signal.